The proposed work is designed to elucidate the neural control of the heart. We shall attempt to devlop a quantitative analysis of cardiac autonomic neural activity by simultaneously solving regression equations which describe two or more independent cardiac responses to simultaneous chages in sympathetic and vagal activity. The changes in the various cardiac responses will be determined during a variety of reflex interventions. The levels of sympathetic and vagal activity during such interventions will then be estimated from the regression equation for such responses derived from the data obtained during randomly applied combinations of direct neural stimulations. We shall also attempt to quantify the reflex cardiac responses to carotid sinus and baroreceptor nerves. Factorial design experiments will be applied to derive regression equations for the responses to various types of baroreceptor stimulation, singly and in various combinations. Such data should permi a quantitative assessment of interactions among the various baroreceptors, including the possibilities of spatial and temporal summation conditioning.